Cosalite in Quartz
A search of the gemological literature shows that rock crystal quartz contains a wide variety of unusual and interesting inclusions. Many of those inclusions look alike, so careful analysis is necessary for a positive identification. We recently examined a 16.20 ct cushion-shaped rectangular modified step-cut rock crystal from Kara-Oba, Betpaqdala Plateau, Kazakhstan (figure 1), that was fashioned by Michael E. Gray (Coast-to-Coast Rare Stones, Mendocino, California). Measuring 18.15 × 13.24 × 10.60 mm, it hosted several opaque silvery gray stalks and needles that appeared to have an orthorhombic morphology (figure 2). A fragment cut from the original rough crystal was analyzed by laser Raman microspectrometry and yielded a possible identification as cosalite, a rare orthorhombic lead bismuth sulfide. This was subsequently confirmed by X-ray powder diffraction. The discovery of such a rare mineral as an inclusion in this stone was a pleasant surprise.
